@param slice_urn (string) URN of slice to allocate to
@param credentials (dict) of credentials
@param rspec (string) rspec to allocate
-
+ @param options (dict)
+
+ The PL driver implements here an important option named
+ 'append' that affects the management of slice tags.
+ It is recommended to set this option to True, and to not pass
+ any slice tag in the incoming rspec, in which case
+ you are guaranteed to leave slice tags alone.
+
+ See also http://svn.planet-lab.org/wiki/SFASliceTags
+
"""
interfaces = ['aggregate', 'slicemgr']
accepts = [